There are some precautionary measures you must take before flashing your Phone.

Backup your data, sms, applications, etc by using Titanium Backup

You will need Rooted Android Phone for this to work.

Ensure that you have more then 200MB space available on your phone.

A fully recharged phone is highly recommended.

If you have done all above mentioned steps, now the real game start:

Step 1 to 4 : making your phone to be ready to be flashed.

Step 5 to 8 : flashing of MIUI

Steps to follow:

Step 1: Take out the external sdcard out of Your Phone.

Step 2: Now Connect your Phone to your Personal Computer with a USB cable and copy .zip MIUI File to the root (Parent Directory) of phone’s SD. Once copied, Just Simply disconnect your phone from the PC.

Step 3: Now reboot your phone in to Recovery Mode (Up Volume Key + Home Button + Power Button). You will see some more options and different out look of Recovery Mode it does not look like the non Root (Blue Display).

Step 4: Once you are in recovery Mode, Scroll Down (by using Volume Up / Volume Down key) to Wipe data/factory reset. Now Scroll Down to advanced using Volume Down key / Home Button then Wipe Dalvik Cache, Then Final step is to scroll down to Mounts and Storage Menu then scroll down to format /system and Press Home button.

Step 5: Now its time to really Flash your phone, Scroll down to install zip from sdcard à choose zip from sdcard à MIUI (Any Version which you want to install).

Step 6: Now MIUI will be flashed on your phone, once done flashing you must once again flash (Means Twice in a row) it doesn’t take much time.Step 7: Final and Most important Step you must POWER OFF by scroll down to power off your SGS II instead of rebooting by using power off option in recovery mode.Step 8: Take out Battery from Phone, After 30 Sec put battery back and switch ON your mobile phone.
Once POWERED ON your phone will starts with MIUI Booting LOGO, and you have MIUI ROM running on your Samsung Galaxy S II.

We have personally tested and used this method on our Galaxy S II I9100 and it is confirmed that it’s working.